The present invention relates to planetary speed reducers and, more particularly, to such reducers of the type having an input sun gear, an output ring gear, a fixed ring gear and one or more planetary gears in mating engagement with the sun and the ring gears.
As is well known, very high reduction ratios are obtainable with speed reducers of the aforesaid type. However, to develop an output, the fixed ring gear and the output or movable ring gear must differ in their number of teeth. This requirement has typically necessitated the employment of two different sets of planetary gears; one set of which meshing with the fixed ring gear and the other set of which meshing with the output or movable ring gear. In addition to the added expense of a second set of planetary gears, such an arrangement is difficult and time-consuming to assemble in that each of the planetary gears of one set must be precisely indexed with respect to its corresponding gear of the other set.
As an alternative arrangement, various prior art devices have been proposed whereby a single set of planetary gears mates with both the fixed and the movable ring gear.
One such prior art arrangement is disclosed in U.S. Pat. No. 3,081,648 wherein the fixed ring gear and the movable output ring gear differ slightly in their number of teeth and engage three planetary gears having a single tooth configuration. The teeth of all the gears are fabricated of plastic material permitting an inherent but limited degree of gear tooth flexibility. Further, the teeth on two of the planetary gears are staggered in opposite directions; with those on the third planetary gear remaining essentially straight.
Another prior art device, disclosed in U.S. Pat. No. 3,675,510, also employs resilient gears which permit free floating planetary gears to yield along their diametral dimension to allow assembly in spite of an intentional interference fit between the gears.
A third device is disclosed in prior U.S. Pat. No. 3,792,629. In the apparatus of this patent the speed reducer comprises a stationary housing having secured thereto a sun gear adapted to drive a plurality of planetary gears which perform two functions. One is to cooperate with the sun gear and a fixed ring gear and the other is to retard or advance a movable ring gear that serves as an output. The output motion is obtained by changing the number of teeth on one of the ring gears without changing the pitch diameter thereof. This method of tooth spacing results in a deliberate mismatch of the gears which causes the movable or output ring gear to rotate relative to the fixed ring gear. The omission or addition of but one tooth is stated as preferable. In one configuration one of the ring gears is reduced slightly in pitch diameter to accommodate one less tooth and its corresponding planetary gear face is similarly reduced by one tooth.